This invention relates to conductive paint compositions and more particularly to a composition directly applied to substrate surfaces so as to form an electrically conductive coating thereon.
The application of conductive coatings to various surfaces such as tapes, film, circuit boards and the like is already well known. Often film or tape is coated with adhesive metallic foil to produce a sensing signal during movement. Such film coatings occasionally crack and are responsible for other problems such as jamming of sprocket driving teeth because of foil overlap of sprocket holes. Application of a conductive coating by means of a liquid paint could avoid the sprocket hole overlap problem, but paint compositions heretofore utilized often crack in response to substrate surface flexure and deteriorate rather rapidly with use because of abrasion and wear.
Conductive paint compositions that are directly applied and air dried are, of course, well known for coating surfaces more compatible thereto such as circuit boards. Such a paint is marketed by G-C Electronics of Rockford, Ill., a division of Hydro-Metals, Inc. When such paint is applied it leaves a coating of silver in a liquid binder formed by Butyl Acetate. This paint composition is known to consist of approximately 43% by weight of silver and 57% by weight of Butyl Acetate and has not been heretofore successfully used to conductively coat tapes or film.
One reason discovered for the lack of success in the use of a silver paint on film, is the excessive concentration of the silver in the liquid vehicle. Although Butyl Acetate as the liquid vehicle for the silver is advantageous in that it is not flammable in a dried state and is non-toxic, it is difficult to remove from the film by use of solvents and when its content in the paint composition is increased to thin the paint or decrease the silver concentration, the paint drying time is excessively prolonged.
It is therefore an important object of the present invention to provide an improved conductive paint composition that is particularly useful for marking of film and also useful for coating many other types of substrate surfaces on which it is desired to apply an electrically conductive layer.